It’s a mortal sin in the eyes of Rome whenever:
- You properly understand the Roman teaching, and
- Reject it with full knowledge and consent.
Like me on the papal monarchy and creation and Biblical authority, or several of the Evangelical Catholic (LCMS) members of our forums on the Eucharist, Biblical authority, and presbyter ordination, etc.
Of course, those who properly understand and then, with full knowledge and consent, reject the Roman teaching, do not believe the traditions of Rome have the competence to define something as a sin apart from God, and, thus, see the Roman proclamation of the sinfulness of a certain belief as “absolutely null and utterly void”, if that belief is not condemned throughout the length and breadth of the Fathers, and in Scripture itself, which is the ultimate authority, and the only “rule which rules, and is not ruled [by anything else]” (norma normans non normata, literally, “the norm which norms and is not normed”, as opposed to the “ruled rule”, the norma normata, “the norm which is normed”).
I am being unclear. Those who reject the teaching of Rome to the point of mortal sin (with proper understanding and full knowledge and consent of the dogma/ta being rejected), do not believe that the pronouncements of Rome on said dogma/ta or mortal sin are valid or binding apart from the pronouncements of God, and thus judge themselves not in sin by the norm of the Scriptures.
I think I’m still being unclear, but it’s a difficult thing. Using an analogy, Rome telling an Orthodox Catholic or a Protestant, etc. that Papal monarchy and infallibility are infallible dogmata and must be accepted on pain of mortal sin, is like telling an atheist who had an abortion that she’s hell-bound from the Scriptures. The Orthodox or Protestant believes the authority of the pronouncements of Rome are roughly the same as the Atheist believes the divine authority of the Scriptures to be.